How to get a clean Ambient Occlusion map with Cycles with low samples?
You can use a normal map to blur the images without touching the edges to keep sharp details.

For that use a bilateral blur attach your ambient occlusion map and the normal map in the slot Determinator. You can play with the settings, i suggest you to lower the "Sigma" setting which is too high by default by something like 1.
You can also apply a small blur at the normal map to avoid getting edges too aliased and be closer to an anti-aliasing filter.